Rhesus monkey alloantisera raised by planned immunization were evaluated for lymphocytotoxic antibodies by (1) titration against peripheral blood lymphocytes of selected monkeys; (2) typing of random unrelated monkeys and of monkey families by standard microcytotoxicity methods; (3) absorption analysis. Such methods have resulted during the past year in an increase of RhLA serologically defined antigens which we recognize from 18 to 21 including 13 in the A series and 8 in the B series. Two monkeys have been identified which behave as LD typing cells or homozygotes at the major locus determining MLC stimulation (LD). Evaluation of these typing cells in pedigreed monkey families and in a random population indicated they identify antigens important in MLC stimulation which are different from but closely linked to the serologically defined transplantation antigens. These LD antigens appear to have a phenotype frequency of about 10% and 2% respectively. A panel of antisera raised to identify antigens expressed predominantly on B lymphocytes has been used to type 52 random unrelated monkeys and a computer assisted cluster analysis for identification of highly correlated antisera is underway. BIBLIOGRAPHIC REFERENCE: Maurer, B.A. and Neefe, J.R.: Definition of 18 SD antigens and 2 LD antigens of the RhLA monkey major histocompatibility complexes. Transplant. Proc. 9: 579-583, 1977.